Helford river is just north of Porthkerris (PK) and can be dived on any tide. Launch out of PK, head north and then up the estuary. On the right hand side of the estuary you should see a small green shed. Divers can be dropped off opposite this shed. Alternatively, there is a yellow buoy near the middle of the river. This belongs to the local yacht club and is home to lots of mussels.
At a maximum depth of 12m and no way to get lost this is a perfect first dive for trainees. There is a ton of life to see and it is a nice change from the usual rocky reef or wreck. The highlight is the thornback rays though. Dive in March and you are almost guaranteed to see them.
WARNING: Do not take mussles that are attached to metal, only those that are attached to rocks/natural substrate. Also, SMBs are a must due to the boat traffic on the river.
More information can be found on Divernet.